The present invention relates generally to a fuel delivery unit and more specifically to a delivery unit which includes a gear pump for drawing oil from a supply and pumping the oil to a burner for combustion. More specifically, the invention relates to an arrangement for mounting a gear on a drive shaft of the fuel pump.
In one type of prior gear pump, an inner gear is fixed on a drive shaft and the resulting assembly is telescoped eccentrically into a larger diameter ring gear with a crescent disposed in the eccentrically created space between the gears. One example of a gear pump including a gear set of this type is disclosed in U.S. Pat. No. 3,307,569 wherein the inner gear is fixed on the drive shaft by means of a drive key or pin which extends through a radial bore in the gear to mate with a longitudinal slot formed in the shaft. Surrounding the ring gear is a stationary gear plate which is secured to a pump housing by means of a cover plate spaced slightly from the end of the shaft. With this arrangement, as the shaft is rotated torque is transmitted from the shaft and through the pin to rotate its gears and thereby pump oil.
A similar mounting arrangement is disclosed in U.S. Pat. No. 2,460,649 with the gear splined on a pin. The patent suggests that this arrangement is to allow the gear to move axially relative to the shaft thereby to avoid imparting thrust between the gear and the shaft.
In still another type of prior mounting arrangement, the gear may be mounted on the end of the shaft with a press fit so that drive from the shaft is transmitted to the gear through the press fit connection.
In each of the foregoing prior art arrangements, the useful service life of the fuel pump may be shortened due to wear in the pump between the drive shaft and the inner gear or as a result of oil leakage and wear between the gear set and the cover plate. This latter leakage and wear may be caused by contact between the end of the shaft should the shaft be forced accidently against the cover plate during handling, for instance. Moreover, inasmuch as such pump also typically uses the pumped oil as a lubricant, the wear between the moving parts of the gear pump may be greatly accelerated when pumping low lubricity fuel oils.
Primarily, the damaging wear is caused by relative motion between parts of the coupling such as may occur during starting or stopping of the pump and even during normal running. Additional wear also may be caused by the contamination resulting from earlier wear. In the connection between the inner gear and the shaft, the wear may be observed, for example, on the inside diameter of the inner gear, on the outside diameter of the shaft where it mates with the gear, and at the contact surfaces of the driving key or pin which may be used to transmit torque from the shaft to the gear. In those pumps where the gear is mounted on the shaft by means of a press fit, wear may be observed between the contact surfaces of the gear and the shaft if the torque transmitting capability of the press fit has been exceeded. As might be expected, once this wear begins between these surfaces, the gear may quickly loosen on the shaft thereby rendering the pump inoperative.